Shipping a dress from Canada

Hi everyone! Just wondering if anyone has any experience on the best way to ship your dress (assuming someone buys it through this site) from Canada? For the record, my dress is a big poofy ballgown, and I guess my concern is that I'm going to need HUGE box in order to ship it and the shipping costs will be very high! I know that the buyer is supposed to pay for the shipping, but I don't really understand how you can estimate the shipping cost for them before you have already packaged it up and shipped it, and I would hate for the buyer to be super excited about buying my dress and then have to deal with a ton of extra shipping costs!